An engine system, such as an engine system of a vehicle, may include one or more heat exchangers fluidly coupled to one or more coolant systems. While an engine of the engine system is in an operational mode, a temperature of components within the engine system may increase. Coolant flowing through the coolant loops may absorb thermal energy from the engine and other components of the engine system, and may transfer the thermal energy to the heat exchangers in order to reduce the temperature of the engine and other components. When the engine is adjusted from an operational mode to a non-operational mode, a flow rate of coolant within the coolant systems may be reduced, resulting in an accumulation of coolant at the heat exchangers and within coolant lines of the coolant systems. Residual thermal energy from the heat exchangers and components of the engine system may then increase the temperature of the accumulated coolant above a boiling temperature of the coolant and cause the coolant to boil, which may result in coolant leaking from the coolant systems and/or degradation of the heat exchangers.
To address the problem of residual thermal energy within the engine system, an example method is shown in U.S. Pat. No. 8,069,827, wherein an electric water pump is mounted in an engine compartment and is driven by a drive component after the engine is stopped in order to reduce a likelihood of coolant temperature increasing above the boiling temperature.
However, the inventors herein have recognized potential issues with such systems, including an absence of a method to prevent boiling of coolant in coolant systems outside of the engine compartment, such as a cooling system of a rear axle of a vehicle. Additionally, while such systems may reduce a likelihood of coolant boiling, thermal energy from the coolant is dissipated by the coolant system, thereby wasting energy that may be of use to the engine system during an engine cold-start. For example, a powertrain in a conventional rear wheel drive vehicle includes a rear axle or differential system that may include axle members and gear sets, transmitting power from a drive shaft to the axle members to propel the vehicle. The rear axle gear sets may be lubricated by lubricating oil to ensure smooth operation of the rear axle differential. Viscosity and other fluid properties of the lubrication oil are a function of temperature and affect the efficiency and performance of the rear axle system and thus the vehicle. The rear axle lubricating oil may be less viscous with increasing temperature and may be more viscous with decreasing temperature. For example, at engine cold start the lubricating oil may be cold and hence, more viscous than desired. Conversely, for example, at high engine load the rear axle lubricating oil may be over-heated and may be less viscous than desired. To minimize friction loss and to reduce wear of the rear axle gears, which may result in reduced fuel efficiency, it is desirable to monitor the rear axle lubrication oil temperature and to maintain the lubrication oil temperature within a specified temperature range for optimal lubrication of the rear axle gears while the engine is operating. It is also desirable to increase the rear axle lubrication oil temperature quickly during an engine cold-start in order to bring the rear axle lubrication oil temperature into the specified temperature range.
In one example, the issues described above may be addressed by a method comprising: flowing coolant through a coolant system while an engine is not operating, while adjusting a flow rate of the coolant through a rear axle heat exchanger (RAHX) in response to a rear axle oil temperature. In this way, coolant flows through the rear axle heat exchanger of the coolant system while the engine is not operating in order to reduce a likelihood of the coolant boiling.
As one example, the coolant system includes the rear axle heat exchanger, an exhaust gas heat exchanger, and a thermal storage vessel. While the engine is not operating, a flow rate of coolant through the exhaust gas heat exchanger is adjusted in response to an exhaust gas heat exchanger fluid outlet temperature, and a flow rate of coolant through the thermal storage vessel adjusted in response to a temperature of the thermal storage vessel. In this way, the likelihood of the coolant boiling is decreased by transferring thermal energy away from at least one of the rear axle heat exchanger or the exhaust gas heat exchanger and into the coolant. The temperature of the coolant is thereby increased, and the coolant may flow through the thermal storage vessel in order to transfer the thermal energy from the coolant to the thermal storage vessel, thereby storing the thermal energy within the thermal storage vessel. The stored thermal energy may be retained for later use, such as during an engine cold-start, in order to warm the rear axle lubrication oil to minimize friction loss and reduce wear of the rear axle gears.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.